As a plasma processing apparatus, there is known a plasma processing apparatus of the RLSA (Radial Line Slot Antenna) type in which microwaves are supplied from a radial line slot antenna into a process chamber to generate plasma (for example, Patent Document 1). A plasma processing apparatus of the RLSA type includes a cylindrical container provided with a worktable disposed therein to place a target object thereon, and an antenna unit comprising a slot plate and a waveguide dielectric body to radiate microwaves. The antenna unit is disposed on top of the cylindrical container with a seal member interposed therebetween to seal the interface, thereby making a vacuum chamber.    [Patent Document 1]    WO 98/33362 (for example, FIG. 1 and so forth)